The present invention relates to droplet-on-demand type ink jet printing systems, and more particularly to ink jet printheads for such printing systems which prevent accumulation and growth of air bubbles in the ink reservoirs of the printheads.
It is well known that the printheads for droplet-on-demand type ink jet printers should be free of air pockets or air bubbles for sustained quality printing, for the bubbles restrict the flow of ink to the nozzles when they grow and reach a sufficient size. Not only can the restriction slow the refill of the passageways or channels to the nozzles, but can block the refill and prevent droplet ejection. Although some air bubbles and dissolved air can be tolerated without print quality being impaired, once air bubbles are present, they tend to grow during the printing operation. Therefore, it is highly desirable to remove continually any air bubbles from the ink supply reservoir during the printing operations so that the air bubbles do not accumulate and coalesce into large enough air bubbles to become a problem.
Air is generally removed by priming the printhead at a maintenance station, such as, for example, as disclosed in U.S. Pat. No. 5,404,158. The priming procedure basically sucks ink from the nozzles bringing with it any air bubbles. Even when this deaerating procedure works, it wastes valuable ink which has been purchased by the end user. Also, in U.S. Pat. No. 5,339,102, the attempt to remove air bubbles from the printhead is done by a priming operation while the printhead is capped at the maintenance station. Unfortunately, the withdrawal of ink by priming does not always remove ink flow restricting air bubbles from the printhead reservoirs or adjacent ink supply passageways, with the result that some nozzles are starved of ink and fail to eject droplets.
U.S. Pat. No. 5,946,015 entitled "Method and Apparatus For Air Removal From Ink Jet Printheads" and assigned to the same assignee as the present invention discloses a decompression technique for removing or relocating air pockets from the reservoirs of ink jet printheads. In one embodiment, an ink jet cartridge, after being filled with ink, is subjected to a relatively high vacuum in an evacutable container. In another embodiment, an accessory kit is used to subject the printhead nozzles and cartridge vent to a high vacuum source after the cartridge is installed in the printer. The nozzles have a higher flow impedance than the printhead ink inlet, so that air bubbles, which expand under a vacuum, move from the printhead reservoir to the cartridge where they do not restrict printhead operation and once removed from the reservoir tend not to reappear there.
U.S. Pat. No. 4,788,556 discloses a deaerator for removing gas dissolved in hot melt ink at elevated temperatures from molten ink in a hot melt ink jet system. An elongated ink path leading to an ink jet printhead is formed between two gas permeable membranes. The membranes are backed by air plenums which contain support members to hold the membranes in position. Reduced pressure is applied to the plenums to extract dissolved air from the molten ink in the ink path. Increased pressure can also be applied to the plenums to eject ink from the printhead for purging.
U.S. Pat. No. 5,808,643 entitled "Air Removal Means For Ink Jet Printers" and assigned to the same assignee as the present invention discloses a method and apparatus for removing dissolved air in ink and air bubbles or air pockets from ink passageways in ink jet printer cartridges by use of a permeable membrane tubing member positioned in the ink at a location adjacent the ink inlet of the printer's droplet ejecting printhead. The permeable membrane tubing member is connected to a vacuum source to diffuse air into the vacuum in the tubing member interior. The vacuum source may be by a direct connection to the printer's vacuum priming pump at its maintenance station, a separate vacuum pump, or a vacuum accumulator.
The generation of exsolved gas or air bubbles in the printhead reservoirs is known to be a significant source of print quality defects. This is especially true for silicon die printheads having etched ink reservoirs, because of the reservoir size and shape. Although this problem of air bubble accumulation and coalescence is well known, prior attempts to solve this problem usually involve adding extra apparatus to the printer.